The correct option is Substitution reaction.
Explanation
In organic chemistry, reactions are classified based on the structural changes that occur in the reactant molecules. Alkanes are saturated hydrocarbons containing only single bonds (C-H and C-C). Due to their saturation, they do not undergo addition reactions but primarily undergo substitution reactions.
Detailed Analysis:
- Substitution Reaction (Correct): The chlorination of methane ($CH_4$) involves the replacement of one or more hydrogen atoms by chlorine atoms. This process occurs in the presence of sunlight (ultraviolet light) or at high temperatures. The reaction proceeds via a free radical mechanism.
Reaction: $CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl + HCl$
Since a hydrogen atom is substituted by a chlorine atom, it is a substitution reaction. - Elimination Reaction: This type of reaction involves the removal of atoms or groups from a molecule to form a multiple bond (double or triple bond). This is characteristic of alkyl halides or alcohols, not simple alkanes under these conditions.
- Condensation Reaction: This involves the combination of two molecules to form a larger molecule, typically with the loss of a small molecule like water or ammonia. This does not occur in the halogenation of methane.
- Isomerisation: This involves the rearrangement of atoms within a molecule to form an isomer (same molecular formula, different structure). Chlorination results in a change in chemical composition, not just structure.
Key Takeaway:
Saturated hydrocarbons (alkanes) are generally inert but undergo free radical substitution reactions with halogens (like chlorine and bromine) in the presence of UV light or heat.